The invention relates generally to position and alignment detection apparatus and methods for rotatable equipment. More particularly, the invention relates to axial displacement and alignment detection apparatus and methods for rotatable shafts, and especially coupled shafts.
As is well known, in most rotatable equipment applications in which there is a coupling between two rotatable elements, axial alignment and displacement are important factors that can affect the life of the equipment and operating efficiency of the system. Ideally, of course, coupled rotatable elements such as shafts, for example, are aligned collinearly on a common axis of rotation to minimize the forces applied to the shafts, couplings and load bearings. However, near perfect and constant alignment is difficult if not impossible to achieve in actual day to day operation of such equipment. A certain amount of axial displacement or float is usually designed into a system, and flexible couplings between shafts are commonly used to compensate for shaft misalignment. Shaft misalignment can be caused by numerous factors such as human error when assembling the system components, bearing wear, thermally induced expansion and contraction of the shaft and bearing assemblies, load variations, settling of foundations that support heavy machinery, and so on to name just a few.
Shaft misalignment can be generally categorized into three types: 1 ) axial displacement in which a shaft is displaced in a direction parallel to the shaft axis of rotation; 2 ) angular displacement or skewing in which a shaft is not parallel with a desired axis of rotation, or in which two coupled shafts have nonparallel axes of rotation; and 3 ) parallel or offset displacement in which two shafts have axes of rotation that are not collinear even if they are parallel. Of course, these three general types of axial misalignments can occur in various combinations or all together. Furthermore, two coupled shafts may exhibit relative axial displacement between the shafts.
As an example, a gas turbine engine may be used to turn a drive shaft that is flexibly coupled to a driven shaft that in turn is coupled to a load such as a generator. A coupling shaft may be interposed between the drive shaft and the driven shaft. Misalignment of the shafts can cause excessive wear of the bearings, can apply excessive loads and torque to the shafts, and generally will decrease the operating efficiency of the system.
Although various mechanical features such as flexible couplings can be used to compensate for shaft misalignment, it is desirable that the shaft positions be monitored or else the couplings can be damaged.
Axial displacement of a shaft is relatively easy to detect. For example, U.S. Pat. No. 4,833,405 issued to Richards et al. shows a dual sensor arrangement for detecting teeth axially spaced on a rotating shaft. Phase differences between the sensor signals can be used to indicate axial displacement. However, this arrangement is susceptible to false readings caused by angular and/or offset displacement. As a result, the arrangement is unsuitable for systems in which shaft misalignment other than simple axial displacement occurs; and is particularly unsuitable for systems that utilize coupled shafts. This arrangement also depends on accurate and fixed positioning of the sensors relative to the shaft sensible elements. The need for two sensors also increases the cost of the overall apparatus, and is further hampered by the need to arrange the sensors in such a manner as to prevent mutual inductance between the sensors.
Angular and offset misalignments are more difficult to detect due to the lack of an easily identified reference alignment. Consequently, shaft angular and offset alignment detectors commonly use variations in magnetic coupling strength caused by distance variations between two magnetically or inductively coupled elements. This general approach, however, is susceptible to noise because the alignment information is amplitude encoded in the detector signals. Furthermore, these apparatus can produce false readings due to axial displacement of one or more of the shafts.
The need exists, therefore, for position and alignment detection apparatus and methods that are simpler and less costly than known arrangements, and which can detect angular, offset and/or axial displacement each independent of the other types of misalignment (or compensated therefore), and which can detect relative displacement between two coupled shafts. Such apparatus and methods should also be usable in combination with a calibration and set-up procedure to align shafts in a system.